The compressor in a cooling unit for a building is basically a pump that compresses gaseous refrigerants. It is not conventionally designed to compress liquids, and should liquid refrigerant flow into the pump as it comes back from the evaporator section of the cooling unit, the valves of the compressor could be damaged, and replacement of the compressor would be required. Moreover, liquid refrigerant in a compressor generally causes dilution of the lubricating oil therein, resulting in premature failure of the compressor.
Accordingly, it is conventional practice to attach some type of heater means such as an electrical heater, to the compressor housing, in order to keep the compressor warm enough to attempt to avoid collecting liquid refrigerant in the compressor, but instead maintaining such refrigerant in gaseous form. However, flood back of liquid refrigerant from the evaporator coil can be too great volume for the capacity of the heater. Also after a prolonged shutdown of the cooling unit, the heater has to be energized for a considerable period of time before becoming effective in heating the compressor housing a sufficient degree to transform any liquid refrigerant returned to the compressor, into gaseous form.